Develop Reference

Java Invocation API: Call the C function back from the java code

I have a C (navive) program and a jar file with the main() method. From my native program I am initializing the JVM, and calling the main() method. I have no problems with this, everything is completely fine. But then I wanted to call back a C function from my java code.
The C function is defined in the native code in the same module as the one, that have created the JVM. The header is auto-generated, and the body is as simple as this:
JNIEXPORT void JNICALL Java_eu_raman_chakhouski_NativeUpdaterBus_connect0(JNIEnv* env, jclass clazz)
{
return;
}
So, from the java code I'm calling NativeUpdaterBus.connect0(), continuosly getting an UnsatisfiedLinkError. I have no System.loadLibrary() calls in my java code, because I thought, that there will be no problems calling the native code back from the java code if the target module is (possibly?) already loaded.
Well, maybe my approach is completely incorrect, but I can't see any obvious defects, maybe you could help?
What possibly could help (but I didn't tried any of these approaches, because I'm still not quite sure)
Use a kind of a "trampoline" dynamic library with these JNI methods, load it from the java code, then marshal native calls through it.
Define a java.lang.Runnable's anonymous inheritor, created with jni_env->DefineClass() but this involves some bytecode trickery.
Use an another, less invasive approach, like sockets, named pipes, etc. But in my case I'm using only one native process, so this might be an overkill.
I'm using OpenJDK 11.0.3 and Windows 10. My C program is compiled with the Microsoft cl.exe 19.16.27031.1 for x64 (Visual Studio 2017).

One possibility, as others have already mentioned, is to create a shared library (.dll) and call it from the native code and from Java to exchange data.
However, if you want to callback to a C function defined in the native code in the same module as the one the JVM originally created, you can use RegisterNatives.
Simple Example
C program creates JVM
it calls a Main of a class
the Java Main calls back a C function named connect0 in the calling C code
to have a test case the native C function constructs a Java string and returns it
the Java side prints the result
Java
package com.software7.test;
public class Main {
private native String connect0() ;
public static void main(String[] args) {
Main m = new Main();
m.makeTest(args);
}
private void makeTest(String[] args) {
System.out.println("Java: main called");
for (String arg : args) {
System.out.println(" -> Java: argument: '" + arg + "'");
}
String res = connect0(); //callback into native code
System.out.println("Java: result of connect0() is '" + res + "'"); //process returned String
}
}
C Program
One can create the Java VM in C as shown here
(works not only with cygwin but still with VS 2019) and then register with RegisterNatives native C callbacks. So using the function invoke_class from the link above it could look like this:
#include <stdio.h>
#include <windows.h>
#include <jni.h>
#include <stdlib.h>
#include <stdbool.h>
...
void invoke_class(JNIEnv* env) {
jclass helloWorldClass;
jmethodID mainMethod;
jobjectArray applicationArgs;
jstring applicationArg0;
helloWorldClass = (*env)->FindClass(env, "com/software7/test/Main");
mainMethod = (*env)->GetStaticMethodID(env, helloWorldClass, "main", "([Ljava/lang/String;)V");
applicationArgs = (*env)->NewObjectArray(env, 1, (*env)->FindClass(env, "java/lang/String"), NULL);
applicationArg0 = (*env)->NewStringUTF(env, "one argument");
(*env)->SetObjectArrayElement(env, applicationArgs, 0, applicationArg0);
(*env)->CallStaticVoidMethod(env, helloWorldClass, mainMethod, applicationArgs);
}
jstring connect0(JNIEnv* env, jobject thiz);
static JNINativeMethod native_methods[] = {
{ "connect0", "()Ljava/lang/String;", (void*)connect0 },
};
jstring connect0(JNIEnv* env, jobject thiz) {
printf("C: connect0 called\n");
return (*env)->NewStringUTF(env, "Some Result!!");
}
static bool register_native_methods(JNIEnv* env) {
jclass clazz = (*env)->FindClass(env, "com/software7/test/Main");
if (clazz == NULL) {
return false;
}
int num_methods = sizeof(native_methods) / sizeof(native_methods[0]);
if ((*env)->RegisterNatives(env, clazz, native_methods, num_methods) < 0) {
return false;
}
return true;
}
int main() {
printf("C: Program starts, creating VM...\n");
JNIEnv* env = create_vm();
if (env == NULL) {
printf("C: creating JVM failed\n");
return 1;
}
if (!register_native_methods(env)) {
printf("C: registering native methods failed\n");
return 1;
}
invoke_class(env);
destroy_vm();
getchar();
return 0;
}
Result
Links
Creating a JVM from a C Program: http://www.inonit.com/cygwin/jni/invocationApi/c.html
Registering Native Methods: https://docs.oracle.com/en/java/javase/11/docs/specs/jni/functions.html#registering-native-methods

System.loadLibrary() is essential for the jni lookup to work. You also have a more flexible System.load() alternative.
Make sure that the native method implementation is declared with extern "C" and is not hidden by linker.

Create a shared library in C for java

When I try to use my library.dll with java I have this error :
java.lang.UnsatisfiedLinkError: C:\Users\ledev\Desktop\library.dll: Can't load IA 32-bit .dll on a AMD 64-bit platform
at java.base/java.lang.ClassLoader$NativeLibrary.load0(Native Method)
at java.base/java.lang.ClassLoader$NativeLibrary.load(ClassLoader.java:2424)
at java.base/java.lang.ClassLoader$NativeLibrary.loadLibrary(ClassLoader.java:2481)
at java.base/java.lang.ClassLoader.loadLibrary0(ClassLoader.java:2678)
at java.base/java.lang.ClassLoader.loadLibrary(ClassLoader.java:2643)
at java.base/java.lang.Runtime.loadLibrary0(Runtime.java:876)
at java.base/java.lang.System.loadLibrary(System.java:1875)
at JNI.HelloJNI.<clinit>(HelloJNI.java:6)
Exception in thread "main"
Here is my code main.c :
#include "main.h"
#include <stdio.h>
#include<stdlib.h>
DLL_EXPORT void hello_jni()
{
printf("Hello, JNI !");
}
Here is my main.h :
#ifndef __MAIN_H__
#define __MAIN_H__
#define BUILDING_DLL
#include <windows.h>
#ifdef BUILD_DLL
#define DLL_EXPORT __declspec(dllexport)
#else
#define DLL_EXPORT __declspec(dllimport)
#endif
#ifdef __c
extern "C"
{
#endif
DLL_EXPORT void hello_jni();
#ifdef __c
}
#endif
#endif // __MAIN_H__
And here is my java code :
package JNI;
public class HelloJNI {
static {
System.loadLibrary("library");
}
private native void hello_jni();
public static void main(String[] args) {
new HelloJNI().hello_jni(); // invoke the native method
}
}
This is how I created my dll :
g++ -c -BUILD_DLL main.c
g++ -shared -o library.dll main.o -Wl,--out-implib,libexample_dll.a
I'm working under intellij IDEA for java and none particular IDE for my c code. My os is windows 10.
I don't really know what I have to modifiy to make it works. None of the topics I read helped me (for exemple : Create a valid shared library in C). If you need anything else, just tell me.

Java JVMTI Agent crashes on GetEnv()

I'm at a total loss here. I'm trying to get a JVMTI agent library running but it keeps crashing for some reason.
I've narrowed it down this line:
(*jvm)->GetEnv(jvm, (void**)jvmti, JVMTI_VERSION_1_0);
this is the full code of the agent lib (in C):
#include <jvmti.h>
#include <stdlib.h>
jvmtiEnv* jvmti = NULL;
JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *jvm, char *options, void *reserved)
{
printf("Agent started.\n");
_flushall();
jint err = (*jvm)->GetEnv(jvm, (void**)jvmti, JVMTI_VERSION_1_0);
if (err != JNI_OK)
{
printf("Failed to get JVMTI env!\n");
_flushall();
return err;
}
return JNI_OK;
}
JNIEXPORT jint JNICALL Agent_OnAttach(JavaVM* vm, char* options, void* reserved)
{
return JNI_OK;
}
JNIEXPORT void JNICALL Agent_OnUnload(JavaVM *vm)
{
}
As I tried to isolate what the issue was I wrote a very simple java app to test
this with:
public class Test
{
public static void main(String[] args)
{
System.out.println("Hello from java!");
}
}
If I run this from netbeans with the VM arg -agentpath set to my .dllcontaining the code above, the app seems to crash when it tries to call GetEnv().
I've made sure of the following things:
- The JVM and the dll are both 64bit.
- The library is most definitely being found and loaded (the printf output is visible before the crash.)
I don't know what else could probably be causing this, do I have to link against some JVMTI API lib that I don't know about?
Or could this be an issue with the java installation on my PC?
Thanks

You should be passing address of jvmti to GetEnv() as in:
jint err = (*jvm)->GetEnv(jvm, (void**) &jvmti, JVMTI_VERSION_1_0);

Java JNI slows down unexpected

I'm trying to implement a java wrapper for RCSwitch in a Raspberry Pi. It works fine until the grabbing method reaches the 80th iteration. Then it slows down and I can't figure out why. It needs more than 5 minutes to return with a value.
I tried to figure out the problem, but I'm not out of memory, the raspberry still has more then 300mega. In spite ot this, I tried to run the JVM with the following parameter:-Xms5m -Xmx5m but the program still slowed down at the 80th iteration so I think its not a memory problem. My sender still sends the value, because if I restart the program it's working again until the 80th iteration, so it's not the lack of input data.
Here is the java part of the code:
public class RCSwitchWrapper {
public native int recievedValue(int PIN);
static{System.loadLibrary("RCSwitchWrapper");}
public static void main(String[] args){
RCSwitchWrapper wrapper = new RCSwitchWrapper();
int counter=0;
while(true){
counter++;
int grabbedData = wrapper.recievedValue(2);
System.out.println(counter+" grabbed data: "+grabbedData);
}
}
}
The C++ part of the code:
#include "RCSwitch.h"
#include "RCSwitchWrapper.h"
#include <stdlib.h>
#include <stdio.h>
#include <iostream>
RCSwitch mySwitch;
JNIEXPORT jint JNICALL Java_RCSwitchWrapper_recieveValue(JNIEnv *env, jobject obj,jint PIN){
if(wiringPiSetup()==-1){
printf("wiringpi error \n");
return 0;
}
mySwitch = RCSwitch();
mySwitch.enableReceive(PIN);
while(1){
if(mySwitch.available()){
int value = mySwitch.getReceivedValue();
return value;
}
mySwitch.resetAvailable();
return(-1);
}
}
Now I'm confused and can't think a solution.
Thanks in advance.

Programmatically (in Java) keep OS X from going asleep

OS X Lion still goes to sleep, even if programmatic mouse "wiggles" are being periodically issued from a Java Robot (this is a standard approach to keeping machines from going to sleep and is documented elsewhere in stackoverflow, and it worked for me prior to OS X Lion).
Can anyone suggest an alternative approach, which does not require permission escalation, of keeping the machine awake from a Java application?
((The fallback solution is obviously to change the Energy Saving preferences so that the machine never sleeps, but we'd like to be smarter than that because requiring the machine to stay awake is the exception rather than the rule)).

It turns out that it is no longer possible to do this with pure-Java hacks, and one must resort to implementing a JNI I/O listener, the code of which would contain something like
http://developer.apple.com/library/mac/#qa/qa1340/_index.html
An alternative, if you can tolerate calling a command line tool, is to call pmset noidle in a daemon thread.

Do you need to prevent display sleep or system sleep? If the latter, you can prevent system sleep with disk activity - e.g. read from or write to a file.
Hard disk activity does not prevent display sleep, but it does prevent
system sleep. It is not uncommon for the display to sleep before the
system goes to sleep if both types of sleep are set to occur after the
same length of inactivity.
http://support.apple.com/kb/ht1776
This answer points to Objective-C code that can be used to prevent sleep, using the UpdateSystemActivity call. Could you create a simple JNI wrapper on this which you could invoke as a native method from your Java code?
It also seems possible to disable and re-enable sleep using applescript. Could you invoke such a script using Runtime.exec() from your Java code?

Following JNI solution works for macOS 10.5 / GCC
I. Definition Java Native Interface
// File KalleInterface.java
public class KalleInterface
{
static
{
// java.library.path set to libKalleInterface.dylib
System.loadLibrary("KalleInterface");
}
public static native boolean preventSleep();
public static native boolean allowSleep();
}
II. Generate Equivalent C++ Interface
Assume compliled java class file is ./bin/KalleInterface.class
javah -jni -d "${PWD}" -classpath "${PWD}/bin" KalleInterface
generates a C++ header KalleInterface.h as
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class KalleInterface */
#ifndef _Included_KalleInterface
#define _Included_KalleInterface
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: KalleInterface
* Method: preventSleep
* Signature: ()Z
*/
JNIEXPORT jboolean JNICALL Java_KalleInterface_preventSleep(JNIEnv *, jclass);
/*
* Class: KalleInterface
* Method: allowSleep
* Signature: ()Z
*/
JNIEXPORT jboolean JNICALL Java_KalleInterface_allowSleep(JNIEnv *, jclass);
#ifdef __cplusplus
}
#endif
#endif
III. Implement C++ Interface
#include "esc_util_KalleInterface.h"
#include <IOKit/pwr_mgt/IOPMLib.h>
// IOPMAssertionCreateWithName ...
// kIOPMAssertionTypeNoDisplaySleep prevents display sleep,
// kIOPMAssertionTypeNoIdleSleep prevents idle sleep
//reasonForActivity is a descriptive string used by the system whenever it needs
// to tell the user why the system is not sleeping. For example,
// "Mail Compacting Mailboxes" would be a useful string.
// IOPMAssertionDeclareUserActivity ...
// claims user activity
#ifdef __cplusplus
extern "C" {
#endif
CFStringRef reasonForActivity= CFSTR("User Activity Type");
// NOTE: IOPMAssertionCreateWithName limits the string to 128 characters.
IOPMAssertionID assertionID;
static bool active = false;
/*
* Class: KalleInterface
* Method: preventSleep
* Signature: ()Z
*/
JNIEXPORT jboolean JNICALL Java_esc_util_KalleInterface_preventSleep(JNIEnv*, jclass)
{
IOReturn success = 0;
if (active)
{
return (jboolean)true;
}
//success = IOPMAssertionCreateWithName(kIOPMAssertionTypeNoIdleSleep, kIOPMAssertionLevelOn, reasonForActivity, &assertionID);
//success = IOPMAssertionCreateWithName(kIOPMAssertionTypeNoDisplaySleep, kIOPMAssertionLevelOn, reasonForActivity, &assertionID);
success = IOPMAssertionDeclareUserActivity(reasonForActivity, kIOPMUserActiveLocal, &assertionID);
active =(success == kIOReturnSuccess);
return (jboolean)active;
//Add the work you need to do without
// the system sleeping here.
}
/*
* Class: KalleInterface
* Method: allowSleep
* Signature: ()Z
*/
JNIEXPORT jboolean JNICALL Java_esc_util_KalleInterface_allowSleep(JNIEnv *, jclass)
{
IOReturn success = 0;
if (!active)
{
return (jboolean)true;
}
success = IOPMAssertionRelease(assertionID);
active = !(success == kIOReturnSuccess);
//The system will be able to sleep again.
return (jboolean)(!active);
}
#ifdef __cplusplus
}
#endif
IV. Build Shared Library
Command line steps for building libKalleInterface.dylib are
g++ -c -fPIC -I ${JAVA_HOME}/include -I ${JAVA_HOME}/include/darwin -o KalleInterfaceCPP.o KalleInterfaceCPP.cpp
g++ -dynamiclib -framework CoreFoundation -framework IOKit -o libKalleInterface.dylib KalleInterfaceCPP.o -lc
V. Test Class
// File KalleTest.java
public class KalleTest
{
public static void main(String...args)
{
boolean ok1 = KalleInterface.preventSleep();
System.out.println(ok1);
try
{
Thread.sleep(60*10*1000);
}
catch (Exception x)
{
x.printStackTrace();
}
boolean ok2 = KalleInterface.allowSleep();
System.out.println(ok2);
}
}
Assuming compiled class is ./bin/KalleTest.class and shared library is ./libKalleInterface.dylib, test is invoked by
java -Djava.library.path=${PWD} -classpath ./bin KalleTest
Expected behaviour: Mac remains active for 10 minutes, each of preventSleep(), allowSleep() produce result/output true.